Absorbent hygienic products employing fiberized wood pulp have been available for many years. However, the tonnage used for this purpose was relatively modest until the advent of disposable diapers, first for infants and later for incontinent adults. The advent of these products and their worldwide use created an explosion in demand. The basic product leaving the papermill is most usually termed a "fluff pulp". In the United States it is most typically a fully bleached southern pine kraft process pulp produced in relatively heavy caliper, high basis weight sheets. The product is rewound into continuous rolls for shipment to the customer. Since the sheeted product is intended to be later reprocessed into individual fibers, low sheet strength is desirable and typically little or no refining is used prior to sheeting. The requirements for surface uniformity and formation are similarly moderate.
At the customer's plant, the rolls are continuously fed into a device, such as a hammermill, to be reduced as much as reasonably possible to individual fibers. The fiberized product is generally termed a cellulose "fluff". This is then continuously air laid into pads for inclusion in the intended product. U.S. Pat. No. 3,975,222 to Mesek is exemplary of such a process.
There are a number of well recognized problems associated with production of cellulose fluff. The hammermills used for fluff production are very large energy consumers. Further, the fluff may contain significant numbers of fiber bundles usually termed knots or nits. More vigorous defiberizing can reduce the knot content but at the expense of considerable fiber breakage and a high resulting content of very fine dusty material. To offset this problem the pulp mill may add chemical debonders prior to sheet formation. These are usually cationic quaternary ammonium compounds with aliphatic substituents on the nitrogen atom that, in essence, coat the fibers with a hydrocarbon film and interfere with the natural hydrogen bonding tendency of cellulose fibers. A typical debonder might have the formula ##STR1## where R.sub.1 and R.sub.2 are long chain hydrocarbons or aliphatic polyether, R.sub.3 and R.sub.4 may be lower alkyl groups, and X is a salt forming anion. Examples of such compounds are seen in Canadian Patent 1,151,213 and Breese, U.S. Pat. No. 4,432,833. Debonders are effective for reducing energy consumption but create their own problem in the form of significantly poorer water absorption rates and somewhat lower water holding capacity. Considerable research has been expended on overcoming this problem; e.g., as exemplified in May et al., U.S. Pat. No. 4,425,186 and Laursen U.S. Pat. No. 4,303,471. However, as yet no fully satisfactory solution to the problem has been found. Manipulation during the papermaking process; e.g., by low or no wet end pressing or by using a higher headbox consistency can contribute only minimally to reduction in fiberization energy. Thus, there had been an unfilled need for a fluff pulp that can be fiberized with significantly lower energy input without losing the excellent water absorption rate of fluff made from an untreated pulp. The product of the present invention ably fills this need.
Lyness et al., in U.S. Pat. No. 3,998,690, separate a fiber stock into two portions. One is treated with an additive to make it cationic while the other is treated with an additive to make it more anionic. The portions are then recombined. The object is to cause flocculation in order to reduce loss of short fibers and fines. Clays are one of the materials said to increase the negative charge on the anionic portion.
Weisman et al., in U.S. Pat. No. 4,469,746, describe coating fibers with a continuous film of silica to improve hydrophilicity. The fibers themselves may be either a naturally hydrophilic material, such as cellulose, or a hydrophobic polymer such as polypropylene.
Jokinen et al., in U.S. Pat. No. 5,068,009 describe preparation of a cellulosic fluff pulp with improved fiberizing characteristics. This is made by treatment of the cellulose by a cellulolytic or hemicellulolytic enzyme at any time during the pulp making process.
Kobayashi et al., in U.S. Pat. No. 5,489,469, describe a liquid absorbent composite product in which water insoluble hydrophilic fibers and a water insoluble inorganic material are embedded into the surface of water absorbent polymer granules, such as a superabsorbent polymer. The inorganic material is chosen from a broad spectrum which includes alumina, silica, talc, clays, and many others. The fibers may be cellulosic. A sheeted pulp product is apparently not contemplated by these inventors.
Eriksson et al., in U.S. Pat. No. 5,492,759 describe methods of adhering hydrophilic inorganic chemicals to fiber surfaces for production of fluff pulps. Aluminum and iron compounds are suggested. The hydrophilic layer results in a decrease in contact angle and an increase in absorption rate.
Swedish Patent Application 8300460-6 describes making an easily debonded pulp by removal of at least 75% of the fines fraction that would pass through a 200 mesh screen. Up to 20-30% of the furnish is removed prior to and during sheet formation.
Swedish Patent No. 462,918 describes an easily debonded fluff pulp made by depositing very finely ground high alpha cellulose particles on the primary fibers. The particles act as spacers between fibers and prevent strong hydrogen bonding between adjacent fibers.
Chauvette et al., in U.S. Pat. No. 5,562,649 describe a flexible absorbent pulp sheet made by incorporating a debonder and then perf-embossing the product. The material may be used as is as an absorbent layer in absorbent hygienic products.
Vinson et al., in U.S. Pat. No. 5,611,890, describe a low dusting tissue product, useful as bath or facial tissue, which incorporates a particulate filler such as kaolin clay as a softening agent.
Research Disclosure Abstract 93355052 (1993) describes air-laid pads made from talc treated wood pulp useful as oil absorbent products or as a hydrophobic low density pad used as cover stock for diapers or other absorbent products. The talc is said to be distributed in the pad as aggregates, submicron particles, or as coatings on the fibers.
It is common practice to use talc in papermaking in very low percentages; e.g., &lt;1%, as a pitch absorbent.
Mineral fillers have long been used in paper manufacture to lower costs and improve surface smoothness and printing properties. Internal usage may vary from as low as about 3% in products such as newsprint to as high as 30% or greater in magazine stock. This internal use must be differentiated from the use of surface coatings that may also have a high content of mineral products. In common with cellulose fibers, most fillers have a negative surface charge. Thus, filler particles and fibers generally tend to repel each other unless some chemical material is used as a retention aid. Without such an aid fillers are primarily retained by filtration in the web as it is dewatered on the forming wire and, since individual filler particles typically have an average equivalent spherical diameter of only about 1 .mu.m or less, loss into the white water is usually quite high.
Retention aids are primarily charge modifiers. They may be anionic or nonionic but are much more usually cationic materials. Depending on their manner of use, retention aids can act by making the fibers cationic or less anionic, or the filler cationic or less anionic, so there is an electrostatic attraction between filler particles and fibers. More generally the retention aids are very high molecular weight cationic water soluble polymers that act as polyelectrolytes. As such, they act as bridges linking filler particles to fibers. Typically they are polyacrylamides, polyanines, polyethyleneimines, polyamidoamines, or polyethylene oxides.
Retention aids can act in a number of ways besides charge control to increase filler retention. They can be used to attract individual filler particles to the fiber surfaces for opacity improvement More usually they are used in a manner that will cause flocculation of fillers with themselves or with fibrils and fiber fines so that the effective particle size is very significantly increased. As such, the floes are much more effectively retained by filtration within the interstices of the dewatering sheet.
Fillers affect paper sheet properties in several ways. Strength, particularly burst and tensile strength, may be reduced. Opacity, brightness, surface smoothness, and ink holdout are usually increased. Particles primarily adhered to the fiber surfaces interfere with fiber-to-fiber bonding. This increases the opacity due to the increased interfacial area that causes a higher proportion of incident or transmitted light to be scattered.
Although there are exceptions, in general papermaking practice it is usually most desirable to encourage filler flocculation so that the filler is predominantly retained in the interfiber spaces rather than on the fiber surfaces. This is done in part to minimize strength loss from use of the filler. Papers are normally rather highly refined to develop strength and ensure excellent formation. However, never before, to the present inventors' knowledge, have mineral fillers been used in conjunction with fluff pulps, which may be either refined-or unrefined, in order to deliberately effect strength loss so as to reduce debonding energy.